Line of Credit - Additional Information (Detail)	9 Months Ended
	Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Debt Instrument [Line Items]
Maximum ratio of total liabilities to tangible net worth	2
Trailing period used for calculating ratios	12 months
Line of Credit
Debt Instrument [Line Items]
Total availability under line of credit agreement	$ 10,000,000.0
Line of credit borrowings	0	$ 0
Letters of credit outstanding amount	3,100,000
Available borrowing under letter of credit	$ 6,900,000
Company's leverage ratio	0.001
Line of Credit | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Basis spread plus (minus) on variable interest rate	1.25%
Line of Credit | Prime Rate
Debt Instrument [Line Items]
Basis spread plus (minus) on variable interest rate	(0.50%)